Wireless communication technologies continue to evolve to meet the demand for increased data throughput. This is addressed on many levels with different approaches including higher order modulation, multiple-input and multiple-output (MIMO), scheduling, increased bandwidth, and so on. In particular, higher carrier frequencies than commonly used today have attracted a lot of interest, as there are larger blocks, e.g. up to several GHz, of continuous spectra available. Especially, the use of radio frequencies in the Extremely High Frequency (EHF) band seems interesting, i.e. frequencies in the range from 30 to 300 GHz. Radio waves in this band have wavelengths from ten to one millimeter, and thus the band is also called millimeter band or millimeter wave, abbreviated as the mmW frequency band. A mmW-based air interface is considered to be one important component of a forthcoming 5G standard. Already today, there is an amendment to the Wi-Fi standard, 802.11ad, which specifies operation in the 60 GHz range with a channel bandwidth of 2.16 GHz.
For the purpose of up-conversion of signals from baseband or intermediate frequencies to said higher carrier frequency and down-conversion from said higher carrier frequency to baseband or intermediate frequencies there is a need for a circuit for generation of a local oscillator (LO) signal, typically in the form of a phase-locked loop (PLL). The PLL in turn requires a reference signal, usually obtained from a crystal oscillator (XO).
As the carrier frequency and signal bandwidth are increased, the duration of frame structure elements is preferably reduced correspondingly, especially when taking a larger leap in carrier frequency, e.g. from 5 GHz to 60 GHz. This has the advantage of giving lower latency in the transmission as well keeping the smallest non-divisible transmission unit (for example an Orthogonal Frequency Division Multiplexing (OFDM) symbol) reasonable sized in terms of number of bits it holds. For example, 802.11n/ac operates at the 2.4 and 5 GHz ISM (industrial, scientific and medical) bands, and for those carrier frequency ranges one OFDM period is 3.2 μs in length excluding guard interval. For 802.11ad operating in the 60 GHz range, the OFDM period is 0.194 μs.
With smaller transmission units, the ability of various blocks in a transceiver (including phase-locked loops and crystal oscillators) to power up and down fast needs to increase correspondingly, especially in battery-operated equipment where powering up and down blocks as needed is a prerequisite for reasonable battery time.
A phase-locked loop operating at cellular frequencies, say around 2 GHz, may take some 100 μs to reach a stable output frequency from cold start. A crystal oscillator may take several milliseconds. In LTE (Long Term Evolution), one OFDM period is roughly 67 μs, and 14 of those compose a sub-frame of 1 ms (including cyclic prefix). Thus, the power up time for the PLL and XO is roughly within the range of an OFDM symbol and sub-frame.
For PLLs operating in the mmW range it can be argued that also the frequency of the crystal oscillator should be increased from that typically used in today's cellular equipment (e.g. 52 MHz), the reason being that the phase noise of the XO gets “amplified” as 20 log 10(fLO/fXO). There exists crystal oscillators operating at up to around 500 MHz based on crystal resonators. A positive side effect is that the power up time will decrease.
The duration of transmission units (e.g. OFDM symbol and frame duration) will decrease substantially for forthcoming 5G mmW-based transmission schemes. Keeping the power-up transition time of today's solution will cause the energy drawn by the XO during start-up to surpass that of the energy used during actual reception and transmission. In addition to this, as the crystal oscillator needs to operate at a higher frequency it will also draw more power thus eroding the short power up time of a high frequency crystal oscillator. For example, a ˜500 MHz XO can draw 5-10 times the current of a ˜50 MHz XO depending on phase noise requirements, start-up time etc. In other words, the problem with existing solutions is that power-up time of crystal oscillators is too high.
Another problem is that crystal oscillator architectures often suffer from the risk of parasitic oscillation.
A differential crystal oscillator circuit is described in US 2010/026402. This circuit uses a differential pair of transistors operating to produce a differential output over a crystal so that an oscillation frequency is established at the differential output of the circuit.
Some solutions have been suggested with the purpose of reducing the start-up time of a crystal oscillator. Some circuits have different biasing conditions for start-up and regular operation, while others try to improve start-up of oscillation by temporarily increasing negative resistance of a feedback circuit. However, such solutions have only shown a limited effect that is not sufficient for the high frequencies described above.
It has also been suggested to inject a current pulse into the crystal causing it to start oscillating. Although this has a certain effect on the start-up time, it is still far from being sufficient.
US 2005/083139 describes a system in which a single ended oscillator circuit can operate in two configurations. In a first configuration, the crystal is connected between a ground terminal and a low-Q wake-up oscillator that applies a series of pulses to the crystal. The low-Q wake-up oscillator can be e.g. an RC Schmidt-trigger based oscillator that wakes up within one pulse. In a second configuration, the crystal is connected in a closed loop with an internal amplifier and operating as a steady state oscillator. This provides a faster wake-up of the crystal oscillator, but the use of a single ended oscillator is less suitable for the present application, e.g. due to problems with phase noise and parasitic oscillations. Single-ended oscillators are normally also more sensitive to interference, and generate more interference than differential ones. Further, this circuit is not suitable for the high frequencies needed in the applications mentioned above.